1. Field of the Invention
The present invention relates to a circuit board, a method for producing a circuit board and, in particular, a circuit board for mechanically and electrically connecting an integrated circuit chip to a support structure, such as a mother-board or the like. Moreover, the present invention relates to an IC BGA (Integrated Circuit Ball Grid Array) package making use of such a circuit board.
2. Description of Prior Art
A schematic cross-sectional view of an existing design of an IC BGA package is shown in FIG. 3.
The IC BGA package comprises an IC chip 10 and a PCB (printed circuit board) 12 for mechanically and electrically connecting the IC chip 10 to an external support, a motherboard or a memory module for example.
The PCB 12 comprises a first dielectric layer 14 and a second dielectric layer 16. A GND/VDD conductive plane 18 is arranged between the first dielectric layer 14 and a second dielectric layer 16. The GND/VDD conductive plane 18 may be a GND conductive plane (GND=ground) or a VDD conductive plane (VDD=power supply potential). Conductive patterns 20 and 22 on the lower surface of the second dielectric layer 10 are electrically connected to the GND/VDD conductive plane 18 by vias 24 and 26. Moreover, the conductive pattern 20 is connected to a GND/VDD pad (not shown) on the IC chip 10 by a bond wire 28. The conductive pattern 22 is provided with a solder ball 30.
In addition, a conductive pattern 32 forming a signal trace is provided on the lower surface of the second dielectric layer 16. The signal trace 32 is connected to a signal pad (not shown) on the IC chip 10 by a bond wire 34. Moreover, the conductive pattern forming the signal trace 32 is provided with a solder ball 36.
The solder ball 30 is for connecting the IC BGA package to a GND/VDD plane on a support, while the solder ball 36 is for connecting to a signal plane on a support, e.g., for soldering a packaged IC to the mother board or a memory module PCB. It is to be noted that only two exemplary solder balls 30 and 36 are shown in the schematic cross-sectional view of FIG. 3 while IC BGA packages generally comprise a multitude of solder balls.
As it is shown in FIG. 3, a recess 38 is formed in the PCB 12 through which the bond wires 28 and 34 are connected to the respective pads on the IC chip 10. The recess is generally referred to as bond channel.
The total height of the existing design of the IC BGA package is large due to the size of the solder balls 30 and 36. In addition, the GND or VDD path from the bond wire 28 to the solder ball 30 includes a minimum of two vias 24, 26. Thus, the total power/ground inductance is increased due to the plurality of vias (24 and 26) in the current path.
Up to now, package height could be reduced only by reducing the number of package PCB layers and/or decreasing the dimension of the solder balls and the thickness of the chip itself. A smaller total power/ground inductance could be achieved by a larger number of solder balls for implementing supply connections.
It is the object of the present invention to provide an apparatus for connecting an integrated circuit to a support and an IC BGA package enabling an increased component density while maintaining good electrical properties.
According to a first aspect of the invention, this object is achieved by an apparatus for connecting an integrated circuit to a support, comprising:                a circuit board having a first surface for attaching the integrated circuit and a second surface opposite to the first surface;        recesses provided in the second surface for receiving at least portions of solder balls for electrically and mechanically connecting the circuit board to the support; and solder pads formed within the recesses.        
According to a second aspect, the present invention provides an IC BGA package comprising:                an integrated circuit;        a circuit board having a first surface to which the integrated circuit is attached and a second surface opposite to the first surface;        recesses provided in the second surface for receiving at least portions of solder balls for electrically and mechanically connecting the circuit board to a support; and        solder pads formed within the recesses.        
In preferred embodiments of the present invention, the circuit board comprises a plurality of insulating layers and internal conductive patterns arranged between the insulating layers. The internal conductive pattern can also be referred to as internal conductive layers. The solder pads in some of the recesses may be portions of such internal conductive layers so that solder balls can be directly applied to the internal conductive layers of the circuit board via recesses in one or more of the insulating layers.
A conductive pattern can be formed on the second surface of the circuit board. In preferred embodiments of the invention, an external ground or VDD plane can be implemented by such a conductive pattern provided on the second surface of the circuit board. The recesses may extend through the conductive pattern on the second surface of the circuit board. Solder balls received in some of the recesses may have direct connection to the conductive pattern on the second surface of the circuit board.
The present invention provides for a circuit board and an IC BGA package having solder balls “drowned” into the recesses provided in the circuit board. Thus, the present invention provides a new design of an IC BGA package with reduced dimensions and improved electrical characteristics.
The conventional BGA package (FIG. 3) with a printed circuit board having two conductive layers has usually one layer substantially occupied by signal traces 32 and another layer (VDD or ground) which occupies a substantial array of the circuit board for a better (reduced) power connection inductance. The present invention enables an improved package having a changed order of layers with which the disadvantages caused by the existing design can be overcome. According to the invention, solder balls can be mounted on internal conductive layers through the recesses or voids in the circuit board. Thus, total package height can be reduced, for example by about half of the solder ball diameter depending on the depth of the recesses. In addition, the GND or VDD conductive plane can have direct connection with associated GND or VDD solder balls so that, for the power supply connection, vias are not required anymore. The inductance of the solder balls will be also reduced due to the smaller current path.
In a preferred embodiment of the present invention, an external conductive plane provided on the second surface of the circuit board is effective as an electromagnetic shield between package traces and the surrounding. Moreover, the whole package will have a better thermal connection to the support to which the chip is mounted making use of the circuit board. Such a support may be any printed circuit board, a mother-board or a memory module board, for example.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.